1. Field of the Invention
This invention relates to a method and apparatus for measuring and spatially resolving the temperature of a flame. More particularly, this invention provides a method and apparatus for measuring the temperature profile of a flame based upon ultraviolet and near-ultraviolet light emissions generated by the flame, the results of which can be used to modify the flame operating parameters to reach desired flame characteristics.
2. Description of Related Art
There are currently no commercial devices or methods known to us that are capable of producing a spatial temperature map of a flame utilizing the ultraviolet light emissions generated by the flame. Available temperature sensors can at most provide single-point temperatures.
The problem of measuring the spatial temperature profile of a flame arises, at least in part, from the fact that most combustion processes are not in complete local thermodynamic equilibrium. Because flames do not possess local thermodynamic equilibrium in most cases, spectrum modeling results will not produce temperatures that represent the true temperature of the flame. Computer models that produce molecule radical spectra rely upon the assumption of local thermodynamic equilibrium. Once this condition no longer exists, it is impossible to accurately predict the temperatures within the flame strictly through thermodynamic principles. The chemical reactions occurring within a flame also tend to excite molecules into a radical state. The spectrum formations that these chemical reactions produce cannot be related directly to flame temperatures through modeling because they are not produced through thermal excitation. However, chemiluminescence is indirectly influenced by flame temperatures. Because many of the kinetic reaction rates involved in hydrocarbon combustion are temperature dependent, the rate at which a chemically excited radical is produced is also temperature dependent. The OH, CH and C2 spectra emitted from hydrocarbon flames are actually a product of both thermal and chemical excitation (chemiluminescence). This condition makes it impossible to deduce the temperature of a hydrocarbon flame not possessing local thermodynamic equilibrium through comparison to spectra simulations.
Several U.S. patents have issued in recent years which aim to determine the temperature of flames based upon their ultraviolet light emission. While these patents all teach valid concepts, they are relatively limited in capability and accuracy. U.S. Pat. No. 6,135,760 to Cusack et al. teaches a method and apparatus for characterizing a flame within a turbine or burner using ultraviolet energy, visible energy and/or infrared energy measurements of the flame in which the amplitude of the frequency or wavelength bands that are indicative of an efficient combustion process, such as those that increase with increases in flame temperature, are measured. Also measured is the amplitude of frequency bands that are indicative of inefficient combustion processes, such as those that do not vary, those that increase a relatively small amount, or those that decrease when the flame temperature increases. More particularly, the method comprises the steps of detecting a first amplitude of energy within a first wavelength band of a first width centered about an emission wavelength of a contaminant in the flame, primarily CH or OH, detecting a second amplitude of energy within a second wavelength band of a second width, which is larger than the first width, which second wavelength band is also centered about the emission wavelength of the contaminant, determining a ratio of the first amplitude of energy to the second amplitude of energy, and comparing the ratio to a known threshold to determine the amount of contaminant in the enclosure.
U.S. Pat. No. 6,318,891 B1 to Haffner et al. teaches a method for determining the adiabatic temperature of a flame by detection of the chemiluminescence radiation from the flame emitted by OH and CH radicals using a spectrograph via an optical sensor fiber. The spectrally resolved raw signal of the chemiluminescence radiation is then corrected and compared with a multiplicity of theoretically determined emission spectra, until a theoretical emission spectrum coincides with the chemiluminescence spectrum. The Boltzmann temperature associated with this coinciding emission spectrum is then assigned to the chemiluminescence spectrum, whereby the adiabatic flame temperature is derived from the Boltzmann temperature by correlation.
U.S. Pat. No. 6,350,988 B1 to Brown teaches an optical spectrometer for combustion flame temperature determination which includes at least two photodetectors positioned for receiving light from a combustion flame and having different overlapping optical bandwidths for producing respective output signals, and a computer for obtaining a difference between a first respective output signal of a first one of the at least two photodetectors with a second respective output signal of a second one of the at least two photodetectors, dividing the difference by one of the first and second respective output signals to obtain a normalized output signal, and using the normalized output signal to determine the combustion flame temperature.